The influence of carbonate complexes on the solubility of zirconia: new experimental data

New experiments conducted at pH 9 and 298 K clearly indicate that addition of bicarbonate ions in aqueous solutions increases the solubility of both cubic and monoclinic zirconia by several orders of magnitude. In pure water the solubility of zirconia proved to be very low (about 10−10 to 10−8 M), while in the presence of 0.05 M NaHCO3 it increased to 10−6– 10−5 M, indicating the formation of strong carbonate complexes. Due to the limited number of available data and the restricted experimental conditions (fixed pH) it was not possible to identify the stoichiometry of the complex(es). However, up to a total carbonate concentration of 0.05 M, our data are consistent with the formation of a mononuclear tetracarbonate or pentacarbonate complex. At higher carbonate concentrations basic Zr carbonates are possibly formed. The formation constant fitted for the pentacarbonate complex is at the upper limit of a range previously estimated with the help of data on analogous actinide-carbonate complexes. Although our results are not sufficient to determine reliable thermodynamic data for the complexation of Zr with carbonate, they can be used to set limits to the solubility of inert matrix spent fuel in radioactive waste storage environments. Our measurements indicate a Zr solubility in the order of 10−6 to 10−5 M for typical repository conditions, which is still 2–3 orders of magnitude below silica concentrations measured from the dissolution of nuclear waste glasses.